Environmental issues are of high importance and concern, especially with the ever increasing number and quantity of chemicals being used and shipped throughout the world today. Toxic contamination and chemical spills need to be analyzed for contents before effective measures for clean-up can be made. In many cases time is of the essence to prevent large scale damage.
The chemicals found in many of these contaminating spills often contain polynuclear aromatic hydrocarbons (PHA's) that are intensely luminescent. Such luminescence provides a means for sensitive analysis. For example, an excitation wavelength can be sent via a telephoto lens to a spill and the spectral response from the spill received. Luminescence from any PHA's is readily measured to help locate and partially identify the material. There is extensive data known on the luminescent properties of many materials.
Luminescence occurs when a material is activated by an energy stimulus whereby the molecules are raised to an excited state. When the stimulus ends, these molecules drop back to a less excited state and give off light energy while doing so. Herein, "light" is defined to be substantially the entire electromagnetic spectrum where excitation and luminescence can be found. If the emitted light energy lasts for a short time, the effect is referred to as "fluorescence", and as "phosphorescence" when the emitted energy lasts for a longer duration. In practice these terms are often used interchangeably.
A useful characteristic is the lifetime of the luminescence--this parameter has become a useful tool and has been incorporated into commercial laboratory instrumentation, e.g. see the Perkin Elmer optical instruments using Xenon flash lamps.
Laboratory instruments, usually called spectrofluorometers or fluorometers, use a narrow band (say, 0.5 to 10 nanometers) of uv (ultraviolet) light as an excitation source. The emitted light of a luminescing sample will always be of longer wavelength than the wavelength of the source excitation. This is because lower energy corresponds to longer wavelengths, and due to the law of conservation of energy the emitted light's energy cannot exceed the excitation light's energy. The emitted light will not be of equal wavelength to the excitation light's wavelength (resonance fluorescence), since some of the energy in the system is lost due to molecular collisions and to thermal dissipation. Therefore the emitted light will be of lower energy and hence longer wavelength. The emitted light is selectively filtered according to wavelength, and, then, the intensity of one or more wavelengths is measured via a photodetector. There are many different fluorometers that have been designed for specific functions, for example, as chromatographic detectors, blood analyzers, and the like. These laboratory spectrofluorometers are tapable of selecting the exciting wavelength, usually by a broad wavelength source incident on an angularly positionable diffraction grating that spectrally disperses the light onto an exit slit such that the wavelength of choice passes through the slit to the sample. A luminescent sample emits wavelengths that are received by photodetectors set at right angles to the excitation light beam to prevent direct incidence of the light from the grating on the detectors. A monochromator may be used to select the wavelengths to be measured (or otherwise received) from the sample. From these and related measurements, substances may be identified and quantified. In less sophisticated devices filters may be used in place of monochromators to quantify the expected substance.
Portable fluorometers are commercially available for dedicated functions and do not serve as general purpose fluorometers. The portable units often employ wavelength selective optical filters and usually use mercury source emission lines for excitation. The qualitative and quantitative uses of these devices is very limited. They respond to many fluorescent species without distinguishing among them, and they are not useful for substance identification. Such devices also lack the ability to measure fluorescence in different forms of matter--as is useful for environmental analysis.
Laboratory instruments useful for identifying and quantifying material, e.g. chromatographs, must separate out the majority of the other material before analyzing the remainder. This takes time and skilled effort. For example, a chromatographic analysis to determine if a major pollutant was present in an oil spill involves separating out the 50 to 100 other chemicals which would interfere with the measurement, and such measurements must be done off-line in a laboratory--not at the site or in-situ. Also, such an analysis requires solvents and highly skilled technicians. The above represent significant limitations.
FIG. 1 is a block diagram of a conventional spectrofluorometer. A high pressure xenon UV light source 2 illuminates a scanning excitation monochromator entrance slit 4. The selected wavelength of light exits 6 the monochromator and is focused on a sample 8 to be analyzed. If the sample has luminescent properties, it will emit light with specific spectral characteristics. During the molecular excitation process, the sample can receive light of a narrow-band of wavelengths for quantitative analysis, or the sample can receive light of progressively changing wavelengths for qualitative identification of unknown compounds. The excitation source may be continuous, pulsed by a shutter or by flashing Xenon flash tubes. Perkin Elmer uses such flash tubes for about 20 micro-second flashes synchronized to the power line frequency (60 Hz). This pulsing is useful in order to measure the lifetime of the emissions from the sample. The emissions from the sample illuminate another entrance slit to a measuring, usually scanning, monochromator. The emissions are resolved spectrally and via an exit slit 12 incident on a photodetector for quantification. The photodetector is usually a photomultiplier tube, but photodiode arrays and charge coupled devices (CCD arrays) are becoming popular. The arrays can accept and measure many wavelengths simultaneously. When these arrays are used, care must be taken to calibrate the position of the array such that specific wavelengths of interest strike known diodes since the light received may contain a contiguous range of wavelengths. Once a selected diode in the array receives a selected wavelength, the next diode, and all the remaining diodes will receive different, but known, wavelengths. The physical separation between the diodes and the optical system determines the wavelength separation between two adjacent diodes. When the photomultiplier tube or a single photodiode is used for the detector, an emission spectrum requires some type of scanning of the monochromator's dispersing elements (e.g. moving a diffraction grating) and taking measurements as each wavelength is incident upon the detector. If an excitation spectrum is needed then there must be some type of scanning of the excitation monochromators.
The information from a spectrofluorometer is presented usually in the two dimensional graph of excitation wavelength vs. emission intensity, or emission wavelength vs. emission intensity. Three dimensional graphs can be produced with sophisticated software. In such cases, the data are fed into a computer to assimilate, normalize or otherwise prepare the data and then display that data. Mechanical monochromators are limited for use with such data presentations since generating the data with mechanical monochromators is tedious and time consuming. There is a continuing need (and limitation of prior art) to integrate all such luminescence information for compound identification.
Expert systems have been developed in recent years. Such systems accumulate large databases and apply sophisticated software for "expertly" performing a given task. There is a need to improve the speed of accumulating such data.
Photo arrays have allowed the detection side of spectrofluorometers to be simplified and made faster by obviating the mechanical scanning of the diffraction grating. But, such systems are still limited by the required mechanical complexity and slowness of rotating a diffraction grating to obtain multiple excitation wavelength.
It is a principal object of the present invention to provide a fast method of identification and quantification of luminescent materials on-site.
It is an object of the present invention to provide for an instrument that provides an excitation spectrum and an emission spectrum with minimal moving parts.
It is another object of this invention to provide apparatus that can spectrally discriminate similar compounds by analyzing the total luminescent properties of such compounds. Such a technique is referred to as multidimensional luminescence (MDL) analysis.
It is an object of this invention to provide apparatus for use in "expert systems".
It is another object of the present invention to provide a qualitative and quantitative spectrofluorometer instrument suitable for measuring matter in solid, liquid or gaseous form.
It is yet another object of the present invention to measure the duration and polarization of the emitted light from a sample under test.
Yet another object of the present invention is to provide all such data to a computer for display in three dimensional plots.
It is yet another object of this invention to provide means to analyze any material that can be made to luminesce, including biological agents.